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Abstract

Background

The relationships between working conditions [job demand, job control and social support];
stress, anxiety, and depression; and perceived quality of life factors [physical health,
psychological wellbeing, social relationships and environmental conditions] were assessed
using a sample of 698 male automotive assembly workers in Malaysia.

Methods

The validated Malay version of the Job Content Questionnaire (JCQ), Depression Anxiety
Stress Scales (DASS) and the World Health Organization Quality of Life-Brief (WHOQOL-BREF)
were used. A structural equation modelling (SEM) analysis was applied to test the
structural relationships of the model using AMOS version 6.0, with the maximum likelihood
ratio as the method of estimation.

Results

The results of the SEM supported the hypothesized structural model (χ2 = 22.801, df = 19, p = 0.246). The final model shows that social support (JCQ) was directly related to
all 4 factors of the WHOQOL-BREF and inversely related to depression and stress (DASS).
Job demand (JCQ) was directly related to stress (DASS) and inversely related to the
environmental conditions (WHOQOL-BREF). Job control (JCQ) was directly related to
social relationships (WHOQOL-BREF). Stress (DASS) was directly related to anxiety
and depression (DASS) and inversely related to physical health, environment conditions
and social relationships (WHOQOL-BREF). Anxiety (DASS) was directly related to depression
(DASS) and inversely related to physical health (WHOQOL-BREF). Depression (DASS) was
inversely related to the psychological wellbeing (WHOQOL-BREF). Finally, stress, anxiety
and depression (DASS) mediate the relationships between job demand and social support
(JCQ) to the 4 factors of WHOQOL-BREF.

Conclusion

These findings suggest that higher social support increases the self-reported quality
of life of these workers. Higher job control increases the social relationships, whilst
higher job demand increases the self-perceived stress and decreases the self-perceived
quality of life related to environmental factors. The mediating role of depression,
anxiety and stress on the relationship between working conditions and perceived quality
of life in automotive workers should be taken into account in managing stress amongst
these workers.

Background

The influence of working conditions on health has been studied extensively over the
last two decades [1-3]. The three most studied characteristics of the working conditions in relation to
health are job demand, job control and social support at work [4]. The interaction of two factors – high job demand and low job control – in the job
strain model [5] may predict adverse health effects on affected workers including fatigue, anxiety,
depression, and physical illness [1]. An extension of this model – the iso-strain model (iso refers to social isolation)
– posits that the most hazardous jobs occur when high job strain is combined with
low levels of social support at work [6]. Most studies using this model have mainly focused on the relationship with cardiovascular
diseases [2,7-9] and also musculoskeletal disorders [10], sickness absence [11], cigarette smoking [12] and general health outcomes such as self-reported health and quality of life (QOL)
[13-15].

Stress, anxiety and depression have been recognized as important outcome measures
in various work environments [16-18]. Plaisier et al. [19] suggested that poor working conditions may be an important precursor of stress and
may, therefore, contribute to the development of depression or anxiety. There are
abundant studies exploring the relationship between working conditions and stress,
anxiety and depression [5,19-24]. Karasek et al. [5] have shown that workers with jobs simultaneously low in job control and high in job
demand reported exhaustion, nervousness, anxiety, and insomnia or disturbed sleep.
Sanne et al. [20] have also shown that job demand, job control and social support were independently
associated with anxiety and depression. Meanwhile, Plaisier et al. [19] reported that job demand predicts the incidence of depressive and anxiety disorders
in both men and women workers, but not for job control (decision latitude) and interaction
between psychological demands and decision latitude.

Stress is often described as being associated with anxiety and depression at workplaces,
and some studies have suggested that stress, anxiety and depression are also related
to poor QOL [25-29]. Mendlowicz and Stein [30] suggested that anxiety disorders have a huge impact on the QOL. Recently, Chen et
al. [26] reported that depressed subjects obviously had lower scores in every subscale of
the health-related QOL than non-depressed subjects and suggested that depressed subjects
were having poor QOL. Ravindran et al. [31] have also reported that major life stresses contribute toward depression, and depressive
illness is often accompanied by marked reduction in QOL. However, the associations
between anxiety and depression with QOL are still unclear due to very limited empirical
data in the literature.

Although various studies have demonstrated the relationship between working conditions
and depression and anxiety [19-21,23,32], awareness of the importance of QOL is also increasingly being recognized as an important
relevant endpoint of outcome measure in diverse health populations including workers
in stressful working conditions. Basically, QOL can be defined as an individual's
perception of his/her position in life in the context of the culture and value systems
in which he/she lives and in relation to his/her goals, expectations, standards and
concerns. It is a broad ranging concept affected in a complex way by the person's
physical health, psychological status, social relationships, and his/her environment
[33]. Several studies have suggested the relationship between working conditions and worker's
QOL [14,15,34,35]. For example, Nasermoaddeli et al. [34] have shown that low job demand is significantly associated with a higher perception
of physical health, psychological status, and social relationship domains; whilst
low job control (decision latitude) is related to the lower perception of the physical
health domain of QOL. Meanwhile, Kudielka et al. [15] suggested that high job demand, low job control and lack of social support at work
exert a significant impact on the self-reported health-related QOL.

Presently, no study has examined the relationship between working conditions and workers'
QOL with stress, anxiety and depression as mediators. Since QOL is associated with
stress, anxiety and depression [25-27,36], it is important to examine the role of stress, anxiety and depression variables
as mediators. The aim of this study is, therefore, to investigate the relationship
between job demand, job control, social support and stress, anxiety, depression and
QOL, where, the effects of stress, anxiety and depression on QOL are also assessed
simultaneously in this relationships using the structural equation modelling approach.

There were four hypotheses that have been developed and require further testing. Firstly,
better working conditions (low job demand, high job control and high social support)
are directly associated with higher perceptions of QOL domains (physical health, psychological
status, social relationships and environment). Secondly, better working conditions
(low job demand, high job control and high social support) are associated with lower
stress, anxiety, and depression levels. Thirdly, lower stress, anxiety and depression
levels are associated with higher perceptions of QOL domains (physical health, psychological
status, social relationships and environment). Finally, a lower stress level is associated
with a lower anxiety and depression level and plays an important role as a mediating
factor in the relationship between better working conditions and the 4 domains of
QOL (physical health, psychological status, social relationships and environment).

Methods

Study design

A cross-sectional study was conducted among male workers in two major representative
automotive assembly plants located in Pahang and Selangor in Malaysia. This study
is part of the Occupational Stress Intervention Study in Petroleum and Automobile
Assembly Plants: Developing and Evaluating Stress Management Program at Workplaces
(OSIS) for a period of three consecutive years beginning from July 2003.

Research protocol

The study protocol was reviewed and approved by the Research and Ethics Committee,
School of Medical Sciences, Universiti Sains Malaysia, Kelantan Health Campus. Permission
to carry out the study was obtained from the Manager of Environmental Health and Safety
Department, and Human Resource Department in each plant. The workers and employers
were also given a written guarantee of confidentiality.

Sample size

The estimation of sample size was performed using the single proportion formula with
95% confidence interval. Sample size calculation was based on the 50% prevalence of
self-reported good or very good health status among Taiwanese male workers using the
WHOQOL-BREF questionnaire [37]. We set the precision at 4% and the calculated sample size was 601. After considering
a 15% non-response, the final calculated sample size was 691.

Recruitment of study subjects

The automotive assembly industry was selected to represent the high income generating
industries in Malaysia. The reference population consists of those workers in the
paint shop and body shop in the automotive assembly plants in Malaysia. The source
population includes workers in an automotive assembly plant in Selangor (plant A)
and Pahang (plant B). The study population comprises 1100 workers for both plants
– 800 in plant A (500 in the paint shop and 300 in the body shop) and 300 in plant
B (200 in the paint shop and 100 in the body shop). The sampling method used for this
study was universal. Permission to carry out the study was obtained from the Manager
of Environmental Health and Safety Department and Human Resource Department in each
plant. Inclusion and exclusion criteria were developed before recruiting the subjects.
The inclusion criteria include male workers who are presently working in the paint
shop and body shop and have worked for at least one year in the industry. Female workers
and those serving less than one year were excluded. Workers were met at their worksites
during working hours. The supervisors were asked to send their workers during the
rest hour to the room set aside for data collection. Recruitment of workers was done
using the list of workers provided by the supervisors; written informed consents were
obtained from the workers participating in the study. Prior to completing the self-administered
questionnaires (JCQ, DASS and WHOQOL-BREF), participating workers were given free
medical check ups as an appreciation for their cooperation. Trained research officers
checked the returned questionnaires onsite to ensure completeness. A total of 767
out of 1100 workers (response rate 69.72%) (521 in plant A and 246 in plant B) were
recruited in the study. After excluding 39 female workers, the final study sample
was 728 workers. However, due to 30 incomplete questionnaires discovered during data
cleaning (missing data), only 698 subjects were included in the analysis.

Working conditions

The validated Malay version of the Job Content Questionnaire [38] was used to measure working conditions that consist of three factors: job demand,
job control (decision latitude) and social support at work based on the job strain
model [1]. In this study, the job demand scale is the sum of 7 items – working fast, working
hard, excessive work, not enough time, conflicting demands, intense concentration,
hectic job and waiting on others; the Cronbach's alpha was 0.74. The job control (decision
latitude) scale – refers to the person's ability to control his or her work activities
[39] – is computed as the sum of 8 items: learning new things, requiring creativity, allows
decision-making, high skill level, decision freedom, task variety, lots of say, and
developing own abilities; the Cronbach's alpha was 0.61. Social support at work is
the sum of 7 items – supervisor pays attention, helpful supervisor, friendly supervisor,
a good organizer, co-workers competent, co-workers interest in me, and co-workers
helpful; the Cronbach's alpha was 0.79. All items were scored on a Likert scale of
1 to 4 (1 = Strongly disagree, 2 = Disagree, 3 = Agree and 4 = Strongly agree; or
1 = Often, 2 = Sometimes, 3 = Rarely and 4 = Never). The scores for job control were
reversed to allow for standardization of scores for job demand and social support.

Stress, anxiety and depression

The validated Malay version of the Depression Anxiety Stress Scales (DASS) questionnaire
was used to measure the negative self-perceived emotional states of stress, anxiety
and depression [40]. Each of the three DASS scales contains 14 items, divided into subscales of 2–5 items
with similar content. The DASS-Stress scale is sensitive to levels of chronic, non-specific
arousal. It assesses difficulty relaxing, nervous arousal, and being easily upset/agitated,
irritable/over reactive, and impatient; the Cronbach's alpha was 0.87. The DASS-Anxiety
scale assesses autonomic arousal, skeletal muscle effects, situational anxiety, and
subjective experience of anxious affect. For this scale, the Cronbach's alpha was
0.82. The DASS-Depression scale assesses dysphoria, hopelessness, devaluation of life,
self-deprecation, lack of interest/involvement, anhedonia, and inertia; the Cronbach's
alpha was 0.87. Subjects were asked to use a 4-point severity/frequency scale (0 =
Did not apply to me at all, 1 = Applied to me to some degree, or some of the time,
2 = Applied to me a considerable degree, or a good part of the time, and 3 = Applied
to me very much, or most of the time) to rate the extent to which they have experienced
each state over the past week. Scores for DASS-Stress, DASS-Anxiety and DASS-Depression
were calculated by summing the scores for the relevant items and converting these
scores into percentile scores [41].

Quality of life

The validated Malay version of the World Health Organization Quality of Life-Brief
Version (WHOQOL-BREF) [42] is a 26-item version of the 100-item instrument of the World Health Organization
Quality of Life (WHOQOL-100) that was developed to provide a short form QOL assessment
concerned with the meaning of different aspects of life to the respondents, and how
satisfactory or problematic are their experiences with these factors. It is a self-reported
questionnaire containing four domains namely physical health (7 items), psychological
status (6 items), social relationships (3 items) and environmental conditions (8 items).
A previous validation study has shown that the Cronbach's alphas for the four domains
of the WHOQOL-BREF were satisfactory (physical health = 0.80, psychological status
= 0.64, social-relationships = 0.65 and environmental conditions = 0.73). All items
were scored on a Likert scale of 1 to 5 (1 = Very poor, 2 = Poor, 3 = Neither poor
nor good, 4 = Good, and 5 = Very good; 1 = Very dissatisfied, 2 = Dissatisfied, 3
= Neither satisfied nor dissatisfied, 4 = Satisfied, and 5 = Very satisfied; 1 = Not
at all, 2 = A little, 3 = A moderate amount, 4 = Very much and 5 = An extreme amount;
1 = Not at all, 2 = A little, 3 = Moderately, 4 = Mostly, 5 = Completely; 1 = Not
at all, 2 = A little, 3 = A moderate amount, 4 = Very much and 5 = Extremely; or 1
= Never, 2 = Seldom, 3 = Quite often, 4 = Very often and 5 = Always). The scores for
some items were reversed to allow for comparison with other facets. The raw score
of items within each domain is used to calculate the domain score by summing up the
scores of the corresponding items in each domain. The domain score is then converted
to a transformed score (ranging from 4 to 20) to enable comparison to be made between
domains composed of unequal number of items. Domain scores were scaled in the positive
direction, i.e. a higher score denotes a higher QOL.

Statistical analysis

The data were analysed using the SPSS version 12.0.1 [43] and Analysis of Moment Structures (AMOS) version 6.0 [44,45]. A structural equation modeling (SEM), a type of multivariate analysis, was applied
to confirm the theoretically built model that includes the domains of working conditions,
stress, anxiety and depression, and consequently, QOL domains. In the first step,
the model was designed and fitted with a well-defined research question. Secondly,
the estimation and their significant levels for each parameter were obtained. Thirdly,
model diagnostics including measures of model fitness and modification indices of
AMOS, were obtained. If indicated, adding correlations between error terms and putting
constraints on parameters were done to improve the model. The chi-square statistic
provides a test of the null hypothesis that the theoretical model fits the data. Jöreskog
and Sördom [46] suggested that the p value for this test of close fit should be more than 0.50. The criteria for model
fit used were relative chi square statistic of less than or equal to 2.0 [45], Goodness-of-Fit Index (GFI) statistic of equal to or greater than 0.95 [46], Adjusted Goodness-of-Fit Index (AGFI) statistic of equal to or greater than 0.90
[45], Comparative Fit Index (CFI) of equal to or greater than 0.90 [47], and Root Mean Square Error of Approximation (RMSEA) of less than or equal to 0.8
[48]. A higher Parsimony Ratio (PRatio) suggests that the model is more parsimonious [49]. Finally, the estimates and significant levels of correlation and regression parameters
from the fit model were presented. Total, direct and indirect effects of working conditions,
stress, anxiety and depression to each QOL domain were calculated using the standardized
regression weights of each pathway.

Results

Characteristics of observed variables

The characteristics of observed variables are shown in Table 1. The mean (SD) age of the workers was 27 (5.9) years. The mean (SD) educational level
was 11.0 (1.4) years. The mean (SD) duration of work and salary were 6.1 (4.4) years
and United States Dollar (USD) 386.1 (276.6), respectively. The mean (SD) job demand,
job control and social support scores were 32.0 (3.8), 66.5 (7.8) and 27.5 (2.7),
respectively. Meanwhile, the mean (SD) stress, anxiety and depression scores were
11.7 (6.8), 8.3 (5.5) and 8.3 (5.8), respectively. The mean (SD) physical health and
psychological status scores were 14.7 (2.0) and 13.7 (1.7), respectively; and that
for the environmental conditions and social relationships were 13.6 (1.9) and 14.8
(2.5), respectively.

Mediating factors

Table 4 shows the total, direct and indirect effects of independent variables on the dependent
variables. Stress, anxiety and depression are important mediators in the relationships
between working conditions (job demand, job control, and social support) and QOL factors
(physical health, psychological status, environmental conditions and social relationships).
Job demand is indirectly related to physical health (mediated by stress and anxiety,
B = -0.07); indirectly related to psychological status (mediated by stress, anxiety
and depression, B = -0.03); both directly and indirectly related to environmental conditions (mediated
by stress, total B = -0.14), and indirectly related to social relationships (mediated by stress, B = -0.05). Job control is directly related to social relationship (B = 0.06). Social support is both directly and indirectly related to physical health
(mediated by stress and anxiety, total B = 0.25); both directly and indirectly related to psychological status (mediated by
stress, anxiety and depression, total B = -0.17); both directly and indirectly related to environmental conditions (mediated
by stress, total B = 0.25) and both directly and indirectly related to social relationships (mediated
by stress, total B = 0.19).

Discussion

Working conditions and quality of life

The relationship between working conditions and QOL have been reported in previous
studies [14,15,34,35]. Important findings of this study reveal that social support is the most directly
associated job factor with all domains of QOL. Hence, an increase in the social support
at work predicts higher perceptions of the four dimensions of QOL. Previous studies
have reported that poor social support is associated with lower perceptions of QOL.
For instance, Cheng et al. [13] reported that poor social support in American women had a significant impact on poor
functional status such as physical functioning, role limitations due to physical health
problems, bodily pain, vitality, social functioning, and role limitations due to emotional
problems, mental health and its decline over time. Other studies have also reported
that low social support is associated with increased blood pressure [50], hyperlipidemia [51] and lower nutrient intake [52].

In addition, the mean age of the study population is relatively young. The matrix
correlation analysis suggests that job control, stress, anxiety, depression and social
relationships increase with increasing age; however, social support and psychological
status decrease with increasing age. Ezoe et al. [53] reported that the grade of psychiatric symptoms measured by the Self-Rating Depression
Scales, General Health Questionnaire, stress due to unsuitable jobs, and most of the
DSM-III-R personality traits decreased with increasing age.

In the present study, job demand was inversely associated with the environmental conditions
domain of QOL; whereas, job control was directly associated with the social relationships
domain of QOL. These findings lend further support to our first hypothesis – favorable
working conditions (low job demand, high job control and high social support) predict
higher perceptions of QOL.

Various studies have also utilized the job demand, job control and social support
model in conceptualizing the working conditions at workplaces in relation to the health-related
QOL. Several studies categorize the job conditions into five: high strain, passive,
low strain, active [14] and iso-strain [54]. Lerner et al. [14] have shown that high strain jobs (high job demand, low job control and low social
support) are associated with poorer perceptions of all five components of the health-related
QOL using the Medical Outcomes Study Short Form 12 Health Survey (SF12) instrument:
physical functioning, role functioning related to physical health, vitality, social
functioning, and mental health. Similarly, Amick et al.[54] reported that high strain jobs are associated with lower vitality, lower mental health,
higher pain, and increased risks for both physical and emotional role limitations
using the SF36 instrument. The present study also found that iso-strain is associated
with an increased risk to the health of workers.

Cheng et al. [13] slightly adjusted the conceptualized model of the working conditions whereby job
demand and job control were individually split into three categories: low, intermediate
and high and social support into two categories: low and high. Based on these categories,
women workers reporting high job demand and low job control (high strain job) were
found to have the worst health status as assessed by a modified version of the SF-36,
whereas those in jobs with high job control and low job demand (low strain job) had
the best health status. Similarly, women workers reporting low job control, high job
demand, and low social support were found to have a greater decline in their physical
health subscales and less improvement in their mental health subscales.

Meanwhile, Kudielka et al. [15] conceptualized the working conditions as consisting of several factors: job demand,
job control, social support and factors related to the effort-reward imbalance (ERI)
model (effort and reward) as primary independent variables and are shown to be significantly
associated with the mental summary score of the Short Form-12 Health Survey (SF12)
including explaining 19% of the variance (R2 = 19.0%). The variance for the working conditions decreases to 13% when accounting
for demographics, socioeconomic status, body mass index, and medical condition. However,
Nasermoaddeli et al., [34] who examined job demand and job control individually, found that job demand is independently
and inversely related to physical health. On the other hand, job control is directly
related to physical health, psychological health and social relationships domain of
WHOQOL-BREF.

Working conditions and stress, anxiety and depression

In the second hypothesis, we expect that poor working conditions (high job demand,
low job control and poor social support) would be associated with higher self-perceptions
of stress, anxiety, and depression. The present study shows that job demand is directly
related to stress; however, job control is not related to stress, anxiety or depression.
Wallgren and Hanse [55] surveyed 167 information technology (IT) consultants in Sweden and found that job
demand is positively related to perceived stress. Their study also found that job
control did not have a significant impact on stress. Whereas, Vanagas and Bihari-Axelsson
[56] found that high job demand combined with low decision latitude have the greatest
impact on stress among 300 Lithuanian General Practitioners.

The present study indicated that social support was inversely related to self-perceived
stress and depression – high social support reduces self-perceived stress and depression.
Thus, the second hypothesis was largely supported by the present study; only job control
was not related to stress, anxiety or depression. Studies have also shown that social
support has a protective effect on the development and prognosis of depression [19,57-59]. For example, Plaisier et al. [19] reported that high social support protected workers from depressive and anxiety disorders
and buffered the unfavorable mental effect of working conditions. However, the beneficial
effect of social support as reported might be limited to alexithymic individuals compared
to the non-alexithymics. Thus, it would be worthwhile to attempt an intervention study
that examines whether subjects who become less alexithymic after attending psychotherapy
sessions that promote emotional awareness and non-verbal communications are able to
utilize social support better than before, as suggested by Kojima et al. [60].

Stress, anxiety and depression and quality of life

Two studies have reported that stress, anxiety and depression were related to poor
QOL [25-29]. The third hypothesis maintains that lower stress, anxiety and depression levels
are related to better QOL. The present study also found that self-perceived stress,
anxiety and depression were inversely related to QOL factors, thus supporting our
third hypothesis. Our data show that stress was inversely related to two QOL factors:
environmental conditions and social relationships. Thus, lower self-perceived stress
predicts higher perception of the QOL related to the environmental conditions and
social relationships.

Our data show that self-perceived anxiety is inversely related to physical health,
thereby suggesting that decreased anxiety predicts higher perception of the QOL related
to physical health. In fact, there is evidence to suggest that anxiety disorders have
a negative impact on QOL [30]. However, there is still a paucity of reports that could corroborate this finding.
Therefore, it might be useful to examine the role of self-perceived anxiety in relation
to QOL among workers.

Our data also show that self-perceived depression is inversely related to the psychological
status domain of QOL: lower self-perceived depression predicts a higher perception
of the QOL related to the psychological status domain. Chen et al. [26] demonstrated the negative effects of depression and physical illness on the scores
of the QOL subscales of SF12 (mental and physical summary scores). Depressed police
officers obviously had lower scores than non-depressed police officers in every subscale
regardless of physical health. Those with physical illness showed lower scores on
the subscale of physical illness, bodily pain, general health, and physical component
summary.

Mediating factors

SEM has supported our fourth hypothesis namely that stress, anxiety and depression
act as mediators for the relationship between working condition factors and QOL factors.
Stress mediates the relationship between job demand and physical health, job demand
and social relationships, and job demand and environment. Similarly, self-perceived
anxiety mediates the relationship between stress and depression, stress and physical
health. Likewise, self-perceived depression demonstrated a mediating relationship
in a number of studies. For example, Friedman et al.[61] predicted that depressed mood would mediate the relationship between social support
and QOL. They found that the direct relationships between satisfaction and social
support and both emotional and functional well-being were mediated by depression:
less satisfaction with social support fostered depressed mood, which adversely impacted
QOL.

We found that self-perceived anxiety was a mediator in the relationship between stress
and depression. This result supports the model linking workload and depression as
proposed by Greenglass et al. [62], who argue that distress at work, as operationalized by cynicism, emotional exhaustion,
and anger is seen to lead towards depressed workload and depression. There are several
methodological considerations that need to be taken into account when interpreting
our results. Since our subjects were male automotive assembly plant workers in Malaysia,
the generalizability of our results is limited. Since the study was cross-sectional,
we cannot determine the directionality of the effects.

Limitation of the study

Several limitations of this study should be noted. The cross-sectional design of our
present study precludes any causal relationship between working conditions and stress,
anxiety, depression and QOL. However, findings from other studies using longitudinal
design, have suggested possible relationships between working conditions and stress,
anxiety, and depression [63-68] and QOL [13,34,54]. To investigate issues of causality, future research could use prospective designs
to replace the subjective responses related to the work environment in individuals
with more objective measures using a job exposure matrix [69], standardized observations [70] and imputation techniques or observer-rating, together with self-report scales [71]. The present study also fully realized that questionnaires are not always reflecting
the "true health". Thus, we suggest that further studies using physiological responses
such as blood pressure and serum cortisol are needed to explore the relationship between
working conditions and health. However, self-report is often the only feasible strategy
to gather information concerning workers' working conditions [72]. Since various instruments were used to measure stress, anxiety and depression such
as the Positive and Negative Affect Schedule (PANAS), Hospital Anxiety and Depression
Scale (HADS), Beck Depression Inventory (BDI), Beck Anxiety Inventory (BAI) and QOL
such as WHOQOL-BREF, Medical Outcomes Study Short-Form Health Survey 12 items (SF-12)
and 36 items (SF-36) and other clinical diagnostic instruments in previous studies
– our findings have limitations to be compared with many previous studies. Despite
that, the mean age (27 years old) of our study sample was low. Kojima et al. [60] reported that the age range between 19 and 39 years are not in high demand nor in
positions of high control in their workplaces, in comparison with those who are in
their 40s or older. This age limitation might explain why our study found that job
demand and job control were not significantly associated with all domains of QOL.
Finally, the sample consisted of a specific group of workers, and one should therefore
be cautious in generalizing the results beyond the study [73]. However, the usefulness of employing specific groups in working life to expand the
understanding of the stress process has been emphasized [71].

Conclusion

To our knowledge, our study is the first to outline the pattern of relationships between
working conditions and stress, anxiety, depression and QOL by using SEM. The use of
SEM permitted simultaneous evaluation of the effects of working condition on QOL;
and stress, anxiety, depression on the QOL within the model. In addition, indirect
effects could be assessed, which would have been impossible with standard regression
methods. The results of the SEM (path analyses) supported the hypothesized model.
Our study suggested that there are significant relationships between job demand, job
control and social support with stress, anxiety and stress and QOL factors. Favorable
job factors, especially high social support, play the most significant effect on all
QOL factors (physical health, psychological status, environmental conditions and social
relationships), stress and depression. Stress, anxiety and depression also have important
roles in relation to QOL factors and should be taken into account in improving the
QOL of workers.

Competing interests

The author(s) declare that they have no competing interests.

Authors' contributions

BNR, BAE and LN contributed equally to the design and conduct of the survey, analysis
of the results, drafting and critical revision of the manuscript. BNR, BAE and LN
read and approved the final version of the manuscript.

Acknowledgements

The authors would like to gratefully acknowledge Prof. Robert Karasek for his permission
to use the Job Content Questionnaire (JCQ) in this study. We would like to acknowledge,
with gratitude, financial support from the IRPA research grant of the Ministry of
Science, Technology and Innovation (MOSTI) (project no: 06-02-05-2079-PR0061/09-03)
in the RMK 8 budget. We would also like to thank Miss Mazalisah Binti Matsah, Dr.
Ahmad Syaarani Bin Yasin, Dr. Nik Khairol Reza Bin Md Yazin for technical assistance
and Mr. Kamarudin Hussin, Research Assistant, for secretarial assistance.

References

Karasek R, Theorell T: Healthy work: Stress, productivity, and the reconstruction of working life. New York , Basic Books; 1990.

Jhun HJ, Cho SI, Park JT: Changes in job stress, musculoskeletal symptoms, and complaints of unfavorable working
conditions among nurses after the adoption of a computerized order communication system.

Plaisier I, de Bruijn JG, de Graaf R, Have MT, Beekman AT, Penninx BW: The contribution of working conditions and social support to the onset of depressive
and anxiety disorders among male and female employees.

Nasermoaddeli A, Sekine M, Hamanishi S, Kagamimori S: Associations between sense of coherence and psychological work characteristics with
changes in quality of life in Japanese civil servants: a 1-year follow-up study.

Knox SS, Theorell T, Svensson JC, Waller D: The relation of social support and working environment to medical variables associated
with elevated blood pressure in young males: a structural equation model.

Plaisier I, de Bruijn JG, de Graaf R, Have MT, Beekman AT, Penninx BW: The contribution of working conditions and social support to the onset of depressive
and anxiety disorders among male and female employees.